Someone posted a picture of the ultra rare Famicom "Godzilla Extra Playing" version on Twitter from an auction long ago. First time I ever saw the cart. Rather than being an officially manufactured not for sale cartridge it appears to be a handcrafted version by the developers themselves for internal use.

I'm a bit sceptical about all those entries in retrofreak. Some are valid, but skaman already found an issue.

Here you can find more info about revisions in the database including Twinbee, but if you take a look at the GameBoy section there are also three "versions" of Kaeru no Tame ni Kane wa Naru, but those are simply the bad dumps from goodgbx which also have bad checksums. Many of those are also real versions but maybe some other hashes are also caused by the device itself.

I'm not at all sure on this but did Seiken Densetsu 2 have more than one mapper variation?
I vaguely recall byuu, being the first to really look at SNES mappers beyond simply "LoROM" and "HiROM", noted that EVO had a rather uncommon mapper. And understandably he wouldn't want to mod a rare and very expensive game for research purposes and someone mentioned SD2 as a far more common game of the same hardware. But I don't remember if it was confirmed all or if there were different mappers of different print runs.